This invention relates to the production of chirped distributed Bragg grating single mode optical fibre filters. A particular, though not exclusive, application for such filters is to be found in the provision of optical fibre dispersion equalisers, as for instance described in GB 2 161 612A, to which attention is directed.
GB 2 161 612A describes a way of making a distributed Bragg grating in a length of photo-responsive single mode optical fibre by launching into it a high-power beam of light, and by using a reflector to set up a standing wave pattern in the fibre. This standing wave pattern produced localised changes in the refractive index of the fibre, resulting in the production of a narrow-band (un-chirped) filter which is selectively reflective at the wavelengths of the light employed to create it. The specification explains that a chirped filter can be synthesised from a number of discrete filters of progressively different centre-wavelengths. It is suggested that those sections can be made either by using light of different wavelengths for the construction of the different sections, or by using light of the same wavelength for constructing each of them, and then altering the periodicity of different ones by different amounts. The method by which it is proposed that this change in periodicity should be brought about is either by elastic stretching of the fibres prior to generation of the grating and then allowing them to relax afterwards, or by performing the operations in the reverse sequence. It is also proposed that, by arranging to set up a stress/strain gradient in a single length of fibre, these processes can be adapted to produce a single-section chirped filter.
An alternative way of making a distributed Bragg grating in a photoresponsive single mode optical fibre is described by G Meltz et al in article entitled: "Formation of Bragg Gratings in Optical Fibres by Transverse Holographic Method", Optics Letters, 1989, 14, (15), pp 823-825. This involves illuminating the fibre from the side with a holographically generated grating fringe pattern. When such fringe patterns are generated with interfering beams of collimated light, the resulting grating is of uniform pitch, though chirped gratings can also be generated by the expedient of using interfering beams of differing divergence.
Both types of method of making distributed Bragg gratings so far described have involved the use of light to create a phase grating in the fibre. A different type of grating, known as a type II grating, is similarly made in single mode optical fibre by lateral holographic illumination, but in this instance the grating fringe pattern is formed much more rapidly, typically using a single short duration pulse from an excimer laser. The creation of such type II gratings is for instance described by C G Askins et al in an article entitled: "Fibre Bragg Reflectors Prepared by a Single Excimer Pulse", Optics letters, 1992, 17 (15) pp 833-835.
With the exception of the interfering beams of different divergence method of coating a chirped distributed Bragg grating optical filter, the above referenced methods of making such chirped filters involve significant additional complexity in comparison with the corresponding above-referenced methods of making uniform pitch filters. A drawback of the interfering beams of different divergence method of making chirped filters lies in the exercise of suitable control over those divergences necessary to create a chirped filter to a specific chirp recipe.